I just picked up some begonias for my garden. Begonias like a loose, humus rich soil mixture. One part loam, 2 part leaf mold, 1 part sand, 1/4 part small lump charcoal, 1/4 part dried cow manure, plus some bone meal at the rate of 1 pt. to the bushel is the optimum preferred soil mix (according to the folks who wrote 10,000 Garden Questions Answered).
Ok, I think I will settle for the less complicated way ~ rich soil, with good drainage and some fertilizer diluted in water and these begonias should be off to a good start. Begonias, can be pinched back when the flowers are wilted. This will prevent the plant from becoming leggy looking.